Dipeptide, oligopeptide, polypeptide, protein. The vocabulary for peptide length looks like a classification and behaves like a set of habits. Knowing where the conventions are firm and where they are loose keeps you from reading precision into a word that does not carry any.
Residue, not amino acid
The first thing the terminology gets right is worth stating. A chain is counted in residues, and a residue is what remains of an amino acid after the bond forms: the free molecule minus the water that condensation removes. A tripeptide contains three residues, which is not the same as containing three amino acids.
The distinction is arithmetic as much as pedantry. A residue weighs about eighteen less than the amino acid it came from, so counting the wrong one throws a molecular weight calculation off by eighteen per position.
The counting prefixes
Short chains take Greek numeric prefixes: dipeptide for two residues, tripeptide for three, tetrapeptide for four, and so on. These are unambiguous, and the naming conventions for amino acids and peptides are set out formally by the joint IUPAC and IUB commission. (Reference: IUPAC-IUB, 1984)
Beyond about ten the prefixes stop being useful and two vaguer words take over. Oligopeptide describes a short chain, conventionally up to around ten or twenty residues. Polypeptide describes a long one. Neither has a boundary anyone enforces.
Where peptide ends and protein begins
The usual convention puts the line around fifty residues. Nothing chemical happens there. The same bond links residue fifty-one as linked residue two, and a chain does not acquire a new property by crossing a count.
Behaviour changes across that range. Short chains tend to stay flexible in solution, while longer ones have enough internal interactions to hold a stable fold. The convention is a rough marker for that shift, and it is applied inconsistently: the same molecule is called a peptide in one paper and a small protein in the next. Our note on amino acids, peptides and proteins covers the underlying distinction.
Direction and numbering
A sequence is written and numbered from the amino terminus to the carboxyl terminus. Residue 1 is the N-terminal one. This is not arbitrary decoration: reading a sequence backwards produces a different molecule that, in most cases, has exactly the same mass, so direction is part of the identity and a mass measurement will not catch the error.
Chemical synthesis runs the other way, building from the C-terminus toward the N-terminus, which is the opposite of how a ribosome works. The numbering follows the sequence, not the order of assembly.
One-letter and three-letter codes
Sequences are written two ways. The three-letter code spells each residue as an abbreviation, Gly-His-Lys, and the one-letter code compresses it to GHK. Short sequences usually appear in three-letter form because it is unambiguous to read aloud; long ones appear in one-letter form because the three-letter version becomes unmanageable.
The one-letter codes are not all first initials, since several residues start with the same letter and only one can have it. That is why tryptophan is W and asparagine is N. A compound name built from one-letter codes, as GHK-Cu is, is quoting a sequence rather than an abbreviation of a longer word.
Residues outside the standard set
Chemistry is not limited to the twenty residues biology uses, and a synthetic sequence can contain units that have no one-letter code at all. Those appear in three-letter form with their own abbreviations, and D-configured residues are marked as such because the mirror image is a different compound with an identical mass.
A sequence written with such residues is therefore longer to state and more informative. Our note on ipamorelin covers a pentapeptide in which three of the five residues sit outside the standard set.
Fragment notation
Hormones cut from a longer precursor carry a numeric range that says which part of the parent they are. A notation such as 7-36 means residues seven through thirty-six of the precursor numbering, and the same hormone can exist as more than one fragment.
A suffix sometimes follows, most often to mark that the chain ends in an amide instead of a free acid. That ending changes the formula and the mass by about a dalton, so the notation is quoting something measurable. Our note on the incretins covers a case where this matters.
Precursor numbering also outlives the cut. A fragment keeps the numbers it had in the parent, so its first residue is rarely residue 1, and two papers describing the same molecule can appear to disagree when one numbers from the precursor and the other from the fragment.
Words that describe shape, not length
Some terms in the same sentence as a length are describing something else entirely. Cyclic says the chain is closed into a ring, which changes the mass by the water lost in closing it; see cyclic and linear peptides. Branched says a side chain carries another chain. Neither is a length.
Peptidomimetic goes further and describes something that is not a peptide at all, built to present a similar shape without the same backbone.
Conjugate is another word in that category. It says something is attached to the chain, a fatty acid or a label, without saying where or how much, and the attachment changes the mass by whatever it weighs.
Why any of this reaches a specification
A specification that says tripeptide is telling you the count, which sets the expected mass range and says something about how the material was made. A three-residue chain is three coupling cycles; a forty-residue chain is forty, with proportionally more opportunity for a deletion sequence that a purity method then has to resolve.
The vocabulary describes the molecule. What is in the vial comes from the measurements on the certificate of analysis and from a batch you can check.
These compounds in the catalog
Frequently asked questions
What is the difference between a residue and an amino acid?
A residue is what remains of an amino acid after the peptide bond forms, which is the free molecule minus a water. A residue therefore weighs about eighteen less, and chain lengths are counted in residues.
What do dipeptide, oligopeptide and polypeptide mean?
Dipeptide and tripeptide give exact counts of two and three residues. Oligopeptide means a short chain, conventionally up to around ten or twenty, and polypeptide means a long one. The last two have no enforced boundary.
Where is the line between a peptide and a protein?
Conventionally around fifty residues, though nothing chemical happens at that count. What changes across the range is whether the chain has enough internal interactions to hold a stable fold, and the convention is applied inconsistently.
Which direction is a peptide sequence written?
From the amino terminus to the carboxyl terminus, with residue 1 at the N-terminal end. Reading a sequence backwards gives a different molecule that usually has the same mass, so direction is part of the identity and mass will not catch the error.
What does a notation like 7-36 mean?
That the peptide is residues seven through thirty-six of a longer precursor, in the precursor numbering. A suffix often follows to mark a C-terminal amide, which changes the mass by about a dalton.
Is cyclic a length term?
No. It describes shape: the chain is closed into a ring, which changes the mass by the water lost in closing it. Branched and peptidomimetic likewise describe structure rather than length.
References
- IUPAC-IUB Joint Commission on Biochemical Nomenclature (1984). Nomenclature and symbolism for amino acids and peptides. Biochemical Journal, 219(2), 345-373.
- Merrifield, R.B. (1963). Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide. Journal of the American Chemical Society, 85(14), 2149-2154.
- Senko, M.W., Beu, S.C., & McLafferty, F.W. (1995). Determination of monoisotopic masses and ion populations for large biomolecules from resolved isotopic distributions. Journal of the American Society for Mass Spectrometry, 6(4), 229-233.
- White, C.J., & Yudin, A.K. (2011). Contemporary strategies for peptide macrocyclization. Nature Chemistry, 3, 509-524.